Active heat sinks which incorporate fans have been described in the art for use in the cooling of heat generating devices such as electronic components and the like. In U.S. Pat. No. 5,288,203 to Thomas a heat sink is shown and described wherein an axial fan is surrounded by a frame support formed by a heat transfer body having a pressure differential surface formed around the perimeter of the fan blades. The pressure differential surface acts like a duct around the fan blades, and is so shown in some of the Figures. As stated the pressure differential surface provides a low pressure region and an axially displaced high pressure region.
With particular reference to the embodiment shown in FIG. 11 of Thomas, a heat transfer body is shown including a number of vertically displaced rings attached in heat conducting relationship by studs with a circular foundation. The rings are described to constitute an optimized heat transfer surface and their internal edges in effect form an air pressure differential surface. This is obtained by placing the rings in such geometric proximity to each other as to be able to form the air pressure differential surface. Ring spacings of the order of between 0.25 and 1 millimeter are taught with 0.7 mm being preferred. The geometry or shape of the rings are further so made as to enhance the axial pressure differential capacity.
The Thomas ring structure emphasizes the presence of a pressure differential surface and thus the need for the proximity of the Thomas propeller tips to the inner edges of the surrounding rings so as to simulate a duct through which air flow is produced by the fan. Thomas teaches the use of fine spacings between the rings and this both diminishes radial air flow therethrough and the extent to which tip vortices can penetrate into the axial ring spacings.
Although Thomas' tight ring spacings reduce induced radial air flow, his fan structure appears to use a ring cage structure as taught by the U.S. Pat. No. 5,292,088 to H. E. Lemont since the operation of the Thomas device approaches a static air pressure characteristic as demonstrated with curve 52 in FIG. 1A in the '088 patent. The emphasis in Thomas on a need for a pressure differential surface requiring a tight ring-to-ring and propeller-to-ring spacings, indicates a failure by Thomas to recognize the significance of propeller tip vortices in the cooling of a heat sink.
In the US patents to Lemont and owned by the assignee of this invention, namely U.S. Pat. Nos. 5,292,088, 5,393,197 and 5,470,202, a ring cage structure is described with a flow augmentation structure. With a ring cage structure as described in these patents the tip vortices from an axial fan are converted to useful airflow and additional mass flow arises. This additional flow enters the spacings between the rings and joins the mass flow from the fan. The '088 Lemont Patent teaches the use of a heat exchanger with fan cooling of rings.
Other patents showing a combination of a fan with a heat sink are U.S. Pat. Nos. 5,297,617 and 5,445,215, which show a fan in a duct to take advantage of turbulent air flow; U.S. Pat. No. 5,299,632 and 5,486,980 and 5,494,098 which show a fan integrated with a fin type heat sink; U.S. Pat. No. 5,309,983 and U.S. Pat. No. 5,335,722, which teach a low profile integrated assembly of a fan with the fins of a heat sink for electronic components; U.S. Pat. No. 5,353,863 for a pentium cooling device; U.S. Pat. No. 5,457,342 which in addition to the use of a fan shows a Peltier type cooling device; U.S. Pat. No. 5,475,564 which also shows a hold down device for connecting a heat sink to a CPU; U.S. Pat. No. 5,504,650 and 5,559,674 which illustrate a variety of different combinations of a heat sink and a cooling fan assembly; U.S. Pat. No. 5,526,875 which shows the placement of a fan at the same level as vertically oriented fins of the heat sink; and U.S. Pat. No. 5,535,094, which shows a heat exchange device with a blower assembly and headers to direct the flow therethrough;
The cooling of heat sinks attached to semiconductor chips is becoming more critical as semiconductors and CPUs such as the Pentium chip generate more heat, that typically is attributable to an increase in processing utilization or an increase in complexity. Generally, performance of such chips is affected by temperature increases and exhibit losses of functions of individual components as the operating temperature increases. It is, therefore, important to prevent temperature increases during operation of a semiconductor chip. Heat sinks, whether these operate without or with a fan (an active heat sink) are, therefore, rated for their thermal resistance, i.e. the amount of temperature rise of the heat sink and thus also encountered in the semiconductor device to which the heat sink is connected to, for each watt of dissipated power, expressed as .degree.C./w. Thermal resistance is a function of the volume and surface area in the heat sink so that any comparison of thermal resistances of heat sinks should assume like volumes or take into account any differences.
The effectiveness or heat transfer capability of an active heat sink is a function of the product of the surface area A, the temperature difference between the heat sink and the fluid (air) moving past the heat sink and a heat transfer coefficient H.sub.c. The heat transfer coefficient H.sub.c in turn depends upon such factors as the geometry of the fluid flow and its velocity past the heat sink surfaces.
The prior art active heat sinks currently used in the industry generally achieve the same best level of thermal resistance, that typically, for like sized heat sink volumes, is in the range of about 1.4.degree. C./w for Pentium or 486 type chips. The thermal resistance level is to some extent dependent upon the speed of the fan used to remove heat from the heat sink, but by increasing the fan speed noise becomes an objectionable side effect. Hence, fan speed cannot as a practical matter be increased indefinitely. Fans also use power which increases at high fan speeds.
One limitation of prior art active heat sinks lies in the fact that the fan occupies precious space and its housing does not contribute to the heat transfer characteristic of the heat sink. Many concepts have been introduced to add surface area to the heat sink by making the housing thermally conductive and this has some benefits.
What is needed, therefore, is an active heat sink with which thermal resistance for an equivalent heat sink volume can be significantly reduced in a manner that does not require more space, more electrical power and does not introduce more fan noise.